TWI433588B - Led lighting device - Google Patents

Description

Light-emitting diode light-emitting device

The present invention relates to an LED (Light Emitting Diode) light emitting device comprising at least a first LED and a second LED powered by a switching power supply.

This device is disclosed, for example, in EP 0716485 A1.

In this arrangement, a switched mode power supply (SMPS) is controlled to supply a constant current to one or more LEDs. The LED light output can then be controlled via pulse width modulation by changing the duty cycle of a bypass switch connected in parallel with the / the LEDs.

In terms of energy efficiency, SMPS is superior to linear power supplies. However, the accuracy of the supplied current may be insufficient for all applications because the load changes, which induces transients in the current feedback system used.

Accordingly, it is an object of the present invention to provide a lighting device of the type mentioned in the opening paragraph in which current accuracy is improved while maintaining a relatively low power consumption.

This object is achieved by a lighting device as defined in claim 1.

More specifically, in the above device, the current through the first LED is controlled by a first bypass switch connected in parallel with the first LED, and the current through the second LED is connected in parallel with the second LED. Connected by a second bypass switch to control, the first LED and the second LED are connected in series to the switching power supply Between a first constant current source, and the switched mode power supply is configured to supply a plurality of different output voltages depending on the state of the first bypass switch and the second bypass switch such that the voltage is dependent on the number of LEDs that are illuminated.

Due to the constant current source, a very well defined current is drawn from the power supply. However, since the output voltage varies depending on the state of the LED, the dissipation in the constant current source can be maintained at a low level. No feedback of actual current is required, which prevents instability and transient problems.

In a preferred embodiment, the first LED and the second LED are configured to be controlled by pulse width modulation such that the first LED and the second LED are simultaneously turned on at the beginning of a pulse width modulation period, and The instantaneous disconnection determined by its respective duty cycle during the pulse width modulation period. The voltage of the switched mode power supply can then be configured to rise to a maximum voltage before the start of the pulse width modulation period. This ensures that a sufficient voltage is supplied at the beginning of the cycle.

The device may include means for driving the output voltage to zero or near zero when all LEDs are turned off. This saves extra power.

The first LED and the second LED can emit light of different colors.

The first LED and the second LED can emit light having green and blue colors and can be connected to the first constant current source via a third LED, the third LED being controlled by a third bypass switch. The first LED and the second LED are further connected to a second constant current source via a fourth LED, the fourth LED is controlled by a fourth bypass switch, and the third LED and the fourth LED emit red light. Or amber light. This allows for a complete, controllable RGBA (red-green-blue-amber) source implemented with a single SMPS.

The embodiments described below will be elucidated and will be readily appreciated by the present invention. And other aspects.

Figure 1 illustrates schematically the bypass control of an LED driven by a prior art constant current source. A constant current source (CCS) 1 is configured to feed a constant current I _const to an LED 3 (light emitting diode). For example, if the LED is a blue emitting LED, the constant current is typically I _const = 700 mA. A bypass switch 5 (typically a MOSFET) is connected in parallel with the LED 3. Using a pulse width modulation (PWM) circuit 7, the bypass switch is fully or fully blocked by PWM control. When the bypass switch 5 is fully conductive, it bypasses the LED 3 so that the LED stops emitting light. Therefore, it is possible to control the luminous flux from the LED by changing the duty cycle of the bypass switch 5. This is done at a switching frequency (eg, 150 Hz or higher) that is high enough to prevent any visible flicker.

It is possible to have two LEDs (each with a bypass switch) sharing a common current source. The two LED/bypass switch combinations are then connected in series with each other. The requirements are: LEDs use the same drive current (for example, red-emitting LEDs and amber-emitting LEDs (350 mA) or blue-emitting LEDs and green-emitting LEDs (700 mA)).

Figure 2 illustrates the available color gamut of an RGB LED configuration with a chromaticity diagram in which the LEDs are controlled by a bypass switch. By using a combination of a red-emitting (R) LED, a green-emitting (G)-emitting LED, and a blue-emitting (B)-emitting LED in combination, a color triangle covering a larger portion of the entire color gamut 11 can be achieved. 9. In order to emit light having a desired color 13, a PWM circuit of each LED is given a predetermined duty cycle such that the correct amount of light is emitted from each LED to produce Want color.

Figure 3a schematically illustrates an LED lighting device in accordance with an embodiment of the present invention. It should be noted that since a switched mode power supply (SMPS) typically has a much better energy efficiency than a linear power supply, it is considered advantageous to use a switched mode power supply (SMPS).

It is possible to use an SMPS as a constant current source by placing a shunt resistor in series with the supplied LED and to adjust the SMPS based on the voltage across the shunt resistor to supply the correct current. This method is described, for example, in EP 0716485 A1. However, in practice, this can be quite complicated due to high current accuracy requirements.

Thus, in one embodiment of the invention, a different scheme is used to control an SMPS. The embodiment shown in Figure 3a has a first LED 15 and a second LED 17, which are connected in series and require the same constant current (e.g., a red-emitting LED and an amber-emitting LED). The first LED 15 is controlled by a first bypass switch 19 that receives a first control signal sw ₁ . The second LED 17 is controlled by a second bypass switch 21 that receives a second control signal sw ₂ . The bypass switches are connected in parallel with the respective LEDs to PWM control the current through the LEDs as described above. LED15,17 by having a system output voltage V _out of the switching mode power supply (SMPS) 8 powered. The SMPS is also PWM controlled but at a much higher frequency (eg, a few hundred kHz). However, the output voltage V _out not by measuring the current through the LED is controlled. Rather, the LEDs are connected in series between the SMPS 8 and a constant current source (CCS) 25.

As long as the output voltage V _out is sufficiently high, i.e., to ensure that the constant current source 25 via a bypass switch LED 15,17 or 19, 21 (when open) to draw a current of a constant and predetermined.

Figure 3b shows an example of one of the constant current sources 25 used in Figure 3a in more detail. The constant current source is known per se and includes a first bipolar transistor T1 and a second bipolar transistor T2 whose base is connected to the base. The first transistor T1 is a diode-coupled (collector-base) and its collector is connected to a reference voltage V _ref via a first resistor R1. The emitter of the first bipolar transistor T1 is connected to ground via a second resistor R2. The emitter of the second bipolar transistor T2 is connected to ground via a third resistor R3. This circuit will draw the constant current I _const at the collector of the second bipolar transistor T2. The constant current is through the first resistor R1, the second resistor R2, the third resistor R3, the reference voltage _Vref, and the base-emitter of the first bipolar transistor T1 and the second bipolar transistor T2. The voltage (which is all constant) is determined.

As is apparent from Figure 3a, as soon as a bypass switch 19, 21 is activated, the voltage drop across the third resistor R3 in Figure 3b will increase. Thus, when the output voltage V _out remains constant, the constant current source to the relatively high power dissipation. Of course, there is a constant current source topology that is different from the topology illustrated in Figure 3b, but this problem remains.

Thus, the SMPS 8 in this embodiment of the invention is adapted to supply a number of different voltages depending on the state of the bypass switches 19,21. This means that the output voltage V _out will depend on the number of the activated LED varies. Typically, the SMPS 8 receives the control signals sw ₁ and sw _{2 of the} bypass switches 19, 21 as input signals. Therefore, when the bypass switches 19, 21 are not running and the two light emitting LED 15,17, the output voltage V _out having a first high voltage. If one of the bypass switches 19, 21 is activated, the output voltage is forced to drop to a second, lower value. If both bypass switches are activated, the output voltage V _out can be changed to 0V or close to 0V, thereby constituting a third value. Therefore, the power dissipation in the constant current source 25 can be maintained at a low level. The SMPS 8 is preferably any type of step down or step down converter.

Figure 4 illustrates an embodiment of the invention in which an RGBA illumination configuration is implemented. In this embodiment, four LEDs (red 29, green 31, blue 33, amber 35) are used, each having a bypass switch 37, 39, 41, and 43, respectively. Each of the bypass switches 37, 39, 41, 43 receives a PWM control signal R, Gr, Bl, A to control the luminous flux of the respective LED.

The red-emitting LED 29 is coupled to a first constant current source 45, which includes a resistor R4 in series with a transistor T3. The LED 35 that emits amber light is coupled to a first constant current source 47 that includes a resistor R5 in series with a transistor T4. The bases of transistors T3 and T4 can be connected to a common voltage reference _Vref . The first constant current source 45 and the second constant current source 47 are preferably identical such that they draw the same current, which may be 350 mA for red and amber LEDs.

The red-emitting LED 29 and its bypass switch 37 and a series connected constant current source 45 are connected in parallel with the amber-emitting LED 35 and its bypass switch 43 and a series connected constant current source 47. Therefore, these circuits can draw a total current of 700 mA, which is a suitable driving current for the LED 31 emitting green light and the LED 33 emitting blue light. Therefore, the green-emitting LED 31 and the blue-emitting LED 33 together with its bypass switches 39, 41 can be combined with the red-emitting LED 29 and the amber-emitting LED 35. The parallel configuration is connected in series. Therefore, it is possible to supply all of the LEDs from one common SMPS 49. In this embodiment, the red-emitting LED and the amber-emitting LED should be controlled in a synchronized manner. However, this requirement is compatible with most color control schemes.

In order to minimize dissipation in the constant current sources 45, 47, the SMPS 49 should be able to treat the red-emitting LED and the amber-emitting LED as one, depending on the number of LEDs that are turned on (0, 1, 2, or 3, Four different output voltages are output because of their synchronous switching. Therefore, the feedback network 51 of the SMPS 49 is adapted to receive the PWM control signals Gr, B1, A/R of the bypass switches 37, 39, 41, 43. Feedback network 51 receives the output voltage V _out, which by a capacitor C1 and an output filter. A voltage divider comprising two resistors R6 and R7 is coupled between the SMPS output and ground and produces a divided feedback voltage Vf. In addition to resistor R6, three resistors R8, R9 and R10 are connected between the SMPS output and resistor R7. The resistor R8 is connected via a switch 53, which is controlled by the PWM control signal Gr of the bypass switch 39 of the green light emitting LED 31. Therefore, if the green LED 31 is turned off, the switch 53 is turned on. Similarly, the resistor R9 is connected via a switch 55 which is controlled by the PWM control signal B1 of the bypass switch 41 of the blue LED 33. The resistor R10 is connected via a switch 57 which is controlled by the PWM control signal A/R of the bypass switch 37 and 43 of the red light emitting LED 29 and the amber light emitting LED 35. Therefore, the voltage divider function of the voltage divider network will vary depending on the number of LEDs that are turned on. For example, if all LEDs are turned on:

If the blue light emitting LED 33 is subsequently turned off, the feedback partial voltage V _{f is} increased to

In the SMPS 49, the feedback partial voltage _{Vf is} compared to an internal reference, and the output voltage is increased or decreased according to this comparison. Thus, as described above, if the feedback voltage V _f partial disconnected when the blue LED is increased, the output voltage V _out decreases. This maintains the voltage across the constant current sources 45, 47 at a low level, thus preventing an increase in power dissipation therein. This means not only saving energy. The constant current sources 45, 47 can also have a lower heat dissipation performance than if the SMPS output voltage is not adjusted in this way.

Of course, this circuit can be adapted to other combinations of LEDs, for example, RRGB (two red-emitting LEDs, one green-emitting LED and one blue-emitting LED), RGB or CMY (green-emitting LED, emission purple) Light LEDs and LEDs that emit yellow light).

Figure 5 illustrates a timing diagram of the configuration of Figure 4. In the upper part, Figure 5 illustrates the output voltage V _out during a PWM period 61, the period 61 may be (e.g.) 2ms in length. The lower portion shows the inversion of the LED PWM control signals Gr, Bl, R, A (as previously described, synchronously switching R and A). When these signals are at level 1, the corresponding LEDs therefore illuminate. In the illustrated example, light having a predetermined color should be emitted. In order to obtain this color, in each PWM cycle, the green-emitting LED emits light during a first time period 63, and the blue-emitting LED emits light during a second longer time period 65, and the red-emitting LED and The LED emitting amber light illuminates during a third longer period 67. For all LEDs, the emission of light starts at the same time, but ends at different points in time. The output voltage V _out 61 with a first initial maximum voltage V _max PWM period begins. When the green-emitting LED is turned off at the end of the first time period 63, the voltage drops to a second, lower voltage. Similarly, when the blue-emitting LED is turned off, the voltage is further dropped to a third level. Finally, when the red-emitting LED and the amber-emitting LED are turned off, the voltage drops to a fourth minimum voltage. At the beginning of the subsequent PWM cycle, all LEDs are turned "on" and the SMPS output voltage is again restored to the first initial maximum voltage _Vmax (i.e., the voltage used when all LEDs are turned on).

Referring to Figures 4 and 5, the circuit shown in Figure 4 can be modified in two ways to improve its functionality.

First, optional components can be provided to ensure that the output voltage is zero or near zero when all LEDs are turned off. This can be, for example, a circuit 69 having a logic AND gate that has its output high when all of Gr, Bl, and A/R go high. This gate can then be used to drive a switch 71 that shorts the resistor R6, thus driving the output voltage to a very low value. This saves some energy consumption.

Second, the member may be provided to ensure that the output voltage has risen before the beginning of each PWM period, so that the output voltage V _out when the initial turns the LED has reached the first maximum voltage V _max. This can be accomplished by adding an optional switch 73 that is configured to disconnect resistors R8, R9, and R10 for a short period of time before the start of the PWM period. This ensures that the constant current source can already draw the correct current from the beginning, thus eliminating potential color errors. If the optional switch 73 is not used, the resistors R8, R9 and R10 are instead directly connected to the voltage dividers of the resistors R6 and R7.

In summary, the present invention is directed to a plurality of LED drive circuits in which each LED is controlled by a bypass switch. The LED is powered by a switched power supply and is coupled to a constant current source to capture a predetermined current through the LED. The switched mode power supply is configured to output different voltages depending on the number of LEDs that are turned on. This is done by supplying a control signal of the bypass switch to the switching power supply. In this way, the power dissipation of the constant current source can be maintained at a low level.

The invention is not limited to the embodiments described above. The invention may be varied in different ways within the scope of the appended claims.

1‧‧‧ Constant Current Source (CCS)

3‧‧‧Lighting diode (LED)

5‧‧‧ Bypass switch

7‧‧‧ Pulse width modulation (PWM) circuit

8‧‧‧Switching Power Supply/SMPS

9‧‧‧Color triangle

11‧‧‧ color gamut

13‧‧‧ desired color

15‧‧‧First LED/LED

17‧‧‧Second LED/LED

19‧‧‧First bypass switch/bypass switch

21‧‧‧Second bypass switch/bypass switch

25‧‧‧ Constant Current Source (CCS)

29‧‧‧Fourth LED/Red LED/Red LED

31‧‧‧Second LED/Green LED/Green LED

33‧‧‧First LED/Blue LED/Blue LED

35‧‧‧ Third LED/Amber emitting LED

37‧‧‧fourth bypass switch/bypass switch

39‧‧‧Second bypass switch/bypass switch

41‧‧‧First bypass switch/bypass switch

43‧‧‧ Third bypass switch/bypass switch

45‧‧‧Constant current source

47‧‧‧Constant current source

49‧‧‧Switched Power Supply/SMPS

51‧‧‧Reward network

53‧‧‧ switch

55‧‧‧Switch

57‧‧‧ switch

61‧‧‧PWM cycle

63‧‧‧First time period

65‧‧‧Second longer time period

67‧‧‧ third longer period

69‧‧‧ Circuitry

71‧‧‧ switch

73‧‧‧Optional switch

C1‧‧‧ output capacitor

I _const ‧‧‧constant current

R, Gr, Bl, A, A/R‧‧‧ PWM control signals

R10‧‧‧Resistors

R1‧‧‧ first resistor

R2‧‧‧second resistor

R3‧‧‧ third resistor

R4‧‧‧Resistors

R5‧‧‧Resistors

R6‧‧‧Resistors

R7‧‧‧Resistors

R8‧‧‧Resistors

R9‧‧‧Resistors

SW1, SW2‧‧‧ control signals

T1‧‧‧First bipolar transistor/first transistor

T2‧‧‧Second bipolar transistor

T3‧‧‧O crystal

T4‧‧‧O crystal

V _f ‧‧‧ feedback partial pressure

V _max ‧‧‧first initial maximum voltage

V _out ‧‧‧output voltage

V _ref ‧‧‧reference voltage / common voltage reference

Figure 1 illustrates schematically the bypass control of an LED driven by a prior art constant current source.

Figure 2 illustrates the available color gamut of an RGB LED configuration.

Figure 3a schematically illustrates an LED lighting device in accordance with an embodiment of the present invention.

Figure 3b shows an example of one of the constant current sources used in Figure 3a in more detail.

Figure 4 illustrates an embodiment of the invention in which an RGBA illumination configuration is implemented.

Figure 5 illustrates a timing diagram of the configuration of Figure 4.

8‧‧‧Switching Power Supply/SMPS

15‧‧‧First LED/LED

17‧‧‧Second LED/LED

19‧‧‧First bypass switch/bypass switch

21‧‧‧Second bypass switch/bypass switch

25‧‧‧ Constant Current Source (CCS)

I _const ‧‧‧constant current

R1‧‧‧ first resistor

R2‧‧‧second resistor

R3‧‧‧ third resistor

SW ₁ , SW ₂ ‧‧‧ control signals

T1‧‧‧First bipolar transistor/first transistor

T2‧‧‧Second bipolar transistor

V _out ‧‧‧output voltage

V _ref ‧‧‧reference voltage / common voltage reference

Claims (6)

A light emitting diode (LED) light emitting device comprising at least a first LED and a second LED (15, 17; 33, 31) powered by a switched mode power supply (8; 49) The current through the first LED (15; 33) is controlled by a first bypass switch (19; 41) connected in parallel with the first LED, through the second LED (17; 31) The current is controlled by a second bypass switch (21; 39) connected in parallel with the second LED, the first LED and the second LED being connected in series to the switched power supply and a first constant current source ( 25; between 47), and wherein the switching power supply is configured to pass depending on the state of the first switch and the second bypass switch of a number of different output voltages supplied (V _out), so that the voltage depends The number of LEDs that are illuminated. The LED lighting device of claim 1, wherein the first LED and the second LED are configured to be controlled by pulse width modulation such that the first LED and the second LED begin in a pulse width modulation period The time is simultaneously turned on, and during the pulse width modulation period, the instantaneous disconnection determined by its respective duty cycle. The LED lighting device of claim 2, wherein the switching power supply is configured to rise to a maximum voltage before the beginning of the pulse width modulation period. The LED lighting device of any one of claims 1 to 3, comprising means for driving the output voltage to zero or near zero when all LEDs are turned off. The LED lighting device of any one of claims 1 to 3, wherein the first LED and the second LED emit light of a different color. The LED lighting device of claim 5, wherein the first LED and the second LED (33, 31) respectively emit a green and a blue light, and the LEDs are connected to the third LED (35) via a third LED (35) The first constant current source (47) is controlled by a third bypass switch (43), wherein the first LED and the second LED are via a fourth LED (29) And connected to a second constant current source (45), the fourth LED (29) is controlled by a fourth bypass switch (37), and wherein the third LED and the fourth LED emit red light or Amber light.